Recently, attention has been drawn to boron neutron capture therapy (BNCT) as a novel cancer therapeutic method utilizing a radioisotope. The boron neutron capture therapy is a therapeutic method in which a boron compound containing boron-10 isotope (10B) is incorporated into cancer cells and the cancer cells are irradiated with low energy neutron (for example, thermal neutron), and thus the cancer cells are locally destroyed by a nuclear reaction which arises in the cells. In this therapeutic method, since it is important to selectively accumulate the boron compound containing 10B in cells of cancerous tissues so as to enhance the therapeutic effect, boron compounds which are selectively incorporated into cancer cells has been developed.
There have been synthesized so far, as a drug used in BNCT, some boron-containing compounds in which boron atoms or boron atomic groups are introduced as a basic skeleton. Examples of the drug used in actual clinical practice include p-boronophenylalanine (BPA) and BSH. Among these drugs, BSH is mainly used for the treatment of brain tumor in the form of a sodium salt, and utility thereof has been confirmed (see, for example, Non-Patent Documents 1 to 8).
On the other hand, it is said that requirements of an amino acid increase so as to assure intracellular metabolism accompanied with abnormal proliferation in cancerous tissues, and that a decrease in selectivity of a membrane transport protein and an increase in a transporting function of an amino acid is accelerated. Therefore, use of an amino acid is one of the options so that a compound may be selectively incorporated into cancer cells. However, there has not yet been reported about BSH into which an optically active α-amino acid with high purity, which is convenient for the incorporation, is introduced.